The present invention relates to apparatus for automatically welding corrugated membrances, and more particularly the invention relates to improvements in and relating to the welding torch driving mechanism of such automatic welding apparatus.
In the lap welding of corrugated membranes which are used in the construction of an LNG tank or LNG vessel, the joining end of each membrane member has a configuration consisting of a series of straight-line and corrugated or curved portions and thus the automatic welding apparatus designed for welding such members is required to serve the dual functions of moving its torch along the welding line as well as in the vertical direction and rotating the torch about an axis which is perpendicular to the two directions. In other words, the welding of a lap fillet welding joint including the curved portions is accomplished by virtue of the resultant motion of the straight line movements in the two directions and the angular movement in the other direction of the torch at a predetermined welding speed and predetermined welding position. In this case, the angular movement of the torch is solely made when welding the curved portions and this angular movement maintains the torch in the same position relative to the joint as in the case of welding the straight-line portions.
With the welding of corrugated membrances, the straight lined portion of the membrane has a flat lower surface and its back surface involves no depressed space, with the result that the heat dissipation is excellent and there is no danger of the membrane being burnt through by the welding heat even if a relatively low welding speed is used. On the contrary, the curved portion of the membrane has a depressed space in its back surface, with the result that the welding heat will be built up in the membrane in correspondence with the volume of the space in the back surface and there is the danger of the membrane being burnt through by the welding heat if a relatively high welding speed is not used. In particular, the small-diameter arcuate portions on the sides and the top portion of the curved section each has a small radium of about 10 mm and consequently the torch must be driven at a high speed in order that the torch may be rotated while welding such a small diameter portion at a desired welding speed.
Known torch driving mechanisms used with the automatic welding apparatus of the above type are generally constructed so that usually an arc gear segment supported on the welding apparatus proper is rotated by a driving pinion gear mounted on the welding apparatus proper and a torch is attached to a head fixedly attached to the arc gear segment. In this case, the rotational speed of the torch is in direct correspondence with the rotational speed of the pinion gear and consequently the pinion gear must be driven at a high speed, thus giving rise to difficulties from the standpoint of rising and falling characteristics at the start and stop of the angular movement and thereby placing a limitation on the welding speed. On the other hand, disclosed in U.S. Pat. No. 4,008,384 is one in which one of a pair of arc gear segments is driven by a pinion gear on the welding apparatus proper to rotate the pair of arc gear segments as a unit and the other arc gear segment is engaged with a pinion mounted, along with a motor, on a torch head which is rotatably attached to the arc gear segments, whereby the two pinion gears are driven simultaneously to rotate a torch at a speed which is substantially two times that obtained with a single arc gear segment. However, this double arc gear type has the disadvantages of requiring an additional motor for the torch head and complicating the arrangements including the wiring of power supply cables to the torch, etc. Another disadvantage is that the head is increased in weight thus requiring an additional driving torque and that the overall weight is increased in the case of a portable type welding apparatus.